What does "LAX" mean?

Lax vowels are also called short vowels:
generally speaking, they are shorter than tense (long) vowels.
(As we shall see, tense vowels have more variable length.)
Another characteristic of lax vowels is that, when stressed,
they are always checked:
that is, they do not occur alone at
the ends of words, but always need a following consonant.
(However, the three weak, unstressed vowels Schwa, KIT and FOOT
do occur at the end of words such as teacher, China, happy, to.
This is discussed later; see WEAK VOWELS in Week 4
)

Lax vowels are always short

This point is sometimes difficult for Icelanders to remember. Refer to 1.4.1
on p. 14 of the booklet English Pronunciation for Icelanders, and read
"lax vowels are always short" on the Web page on vowel length
here